Milk PTA rate of change is greatest in mid lactation
Take Home Message: Selection for increased milk yield and lower somatic cell counts based on 305-d lactation data is resulting in a shift in peak yield, a flatter lactation curve and lower cell counts in mid-late lactation.
The rate of change of milk predicted transmitting abilities (PTAs) at various days in milk (DIM) is greatest around mid lactation. That was just one of the findings of work carried out by the SAC’s Raphael Mrode, who led a team to evaluate genetic trends in terms of changes in PTAs at different days in milk (DIM) for production traits and somatic cell counts (SCC) in the Holstein breed.
The inclusion of SCC in the national profitable life index (£PLI) is based on its relationship with milk computed on a 305-day basis, so the study also looked at what impact such selection had on the relationship between PTAs for milk and SCC at different DIM.
“Genetic progress has been accompanied by a shift in the peak of the genetic lactation curve. And while there has been a decrease in SCC in late lactation, SCC seems to have increased in early lactation,” he told delegates at this year’s British Society of Animal Science annual conference.
“The use of test day model (TDM) for genetic evaluation results in predicted transmitting abilities (PTAs) for any stage of lactation. But in the UK publication and selection of parents are both still based on PTAs computed on a 305-day basis from the random regression coefficients of the TDM.
“It is expected from selection index theory, that selection on 305-day PTAs would place different weights on PTAs at different days in milk (DIM). This could result in changes in the lactation curve of animals over time as a result of genetic progress,” said Dr Mrode, explaining the rational behind his work.
The analysis for production traits involved 6,450,558 cows born from 1974 to 2006 and with about 58 million test day (TD) records. A total of 3,294,634 heifers born 1986 to 2006 with 27,895,096 TD records were evaluated for SCC. Genetic trends were estimated for 305-day and different DIM by regressing PTAs on the year of birth.
The percentage contribution of the trend at each DIM to the overall trend was computed and the average genetic lactation curve was examined over time. Correlations of PTAs for milk and SCC for bulls, with at least 50 daughters for both traits, were computed at 305-day basis and different DIM and then changes in the relationship between both traits over DIM were examined.
“And we found that bull genetic trends for first parity milk yield increased by 36.1kg per year during the period from 1964 to 2003. The rate of change for PTAs, at 120-210 DIM, accounted for 43% of the trend observed on the 305-day basis, with changes for PTAs at 30 DIM accounting only for between 7% and 8%,” said Dr Mrode.
“In general the highest rate of change (11%) was observed for PTAs at 150 and 180 DIM.”
The cow genetic trend for increased milk yield was 38.5g per year on a 305-day basis, with about 11% of the change observed at 150 DIM. The plot of the genetic curve for cows followed the same pattern as bulls.
“In contrast, the rate of change for SCC was in line with DIM in milk, with +36, +1 and -22% of the rate for 305-day basis contributed by SCC PTAs at 30, 210 and 305 DIM.
“The genetic curve for SCC indicates that this linear increase has declined with time. It seems that the reduction in SCC as a result of selection has mainly been due to reduction in SCC at mid to late stages of lactation.
“This is consistent with estimates of correlation between Bull PTAs for milk and SCC. While the correlation of 305-day milk and SCC PTAs was 0.22, the estimate between 305-day milk PTA and SCC PTAs at 30, 150, 270 and 305 DIM were 0.41, 0.30, 0.06 and 0.01.
“This indicates a stronger antagonistic relationship between milk and SCC at early stages of lactation,” added Dr Mrode.
Summary (pdf) Presentation (pdf)
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